Process for Dyeing of Wool or Silk and Their Blends with Indigo

ABSTRACT

A process for pretreatment of wool, silk or blends thereof rendering it receptive for the subsequent step of indigo dyeing. The pretreatment steps comprise in the step of cationization for imparting cationic charge and generating other dye combining sites.

FIELD OF INVENTION

This invention relates to a process for dyeing of wool, silk or theirblends (fibre/yarn/fabric) in tandem with indigo. In particular, thisinvention relates to a pretreatment of wool, silk or their blends inorder to render it receptive to indigo dyeing.

BACKGROUND OF THE INVENTION

The literature on wool discloses that the molecular grid of wool(Keratin) consists of polypeptide chains bound together by salt linkagesand the covalent disulphide (—S—S) linkages. Such a literature furtherdiscloses that the disulphide bond is easily and readily degraded in thepresence of alkali. Alkali disrupts the disulphide bonds and the fissionof disulphide bonds results in the formation of thiol groups, which arevery labile.

It is known that as a natural polyamide or polypeptide, silk Fibroin(degummed silk) exhibits free amino, carboxyl and phenolic substituteswith accessible hydroxyl groups. Silk and wool have a slightly cationiccharacter with an isoelectric point at about pH 5.0. Thus, in an acidicsolution, silk fibroin or wool takes on a positive charge throughabsorption of hydrogen ions. The resulting electric charge can becounter balanced with negatively charged anions of the dye, whilstdyeing with anionic dyes such as acid, metal complex, direct andreactive dyes.

It is known that indigo dyeing is carried out under highly alkaline pHvalues and that leuco indigo has an anionic charge. However, silk andwool have a tendency to lose their cationic property in presence ofalkali. Both fibre and the dye being anionic under alkaline conditions,the net result is loss of dye-fibre affinity, lighter dyeing and poorfastness.

Two of the silk properties that determine its colour behaviour are,firstly, very fine fibre fibrils and, secondly high fiber orientation.These fine fibrils produce a large fiber surface. Such a large fibersurface results in the colour yield being very low and about twice asmuch dye is required for achieving a given dark shade. Further, thefastness is poor. A large fiber surface also leads to a high dye-strikerate even at very low temperature resulting into rapid saturation at thefiber surface and unlevel dyeing. The high orientation of the fibres,which is a barrier to diffusion, causes a very slow rate of further dyeabsorption, after the first rapid saturation.

Further, the surface of silk is very easily chafed by abrasionparticularly when the material is in wet swollen state.

The polypeptide chains of silk fibroin may hydrolyze in boiling water orin steam. This becomes critical in the presence of acids or alkalis.Higher concentrations of alkalis rapidly dissolve silk and wool.Degradation of silk and wool is determined by pH and the type of alkali.The mechanical stress leads to splicing of silk threads, which is severeand irreparable damage.

Thus, it would be apparent that the normal known steps of dyeing of silkcannot be employed for indigo dyeing of silk (and wool) in general.Firstly, indigo dyeing is conventionally carried out under highlyalkaline pH values. Both silk and wool would cease to be cationic underalkaline conditions and would assume anionic character losingelectrostatic affinity for anionic leuco indigo. The result would belighter and skittery dyeing and poor fastness.

The rope/slasher indigo dyeing machine is a long battery of severaldip-squeeze-air modules with pre-wet, Pre-wet washes and post-dye-washoff boxes. The yarn passage through the said machine would be as long as400-500 mtrs. At dyeing speed of for example 18-20 mtrs/minute, the yarnruns through hostile environment; for 25 minutes including long exposureto caustic soda, hot washes and mechanical stresses. All these coulddeteriorate the mechanical properties of silk and wool.

OBJECTS OF THE INVENTION

An object of this invention is to propose a novel process for the dyeingof wool, silk or blends thereof with indigo in conventional dyeingmachines.

Yet another object of this invention is to propose a novel pretreatmentprocess for dyeing of wool, silk or blends thereof with indigo.

Still another object of this invention is to propose a novel process forthe dyeing of wool, silk or blends thereof with indigo, which reducesthe severity of dyeing conditions.

A further object of this invention is to propose a navel process for thedyeing of wool, silk or blends thereof with indigo, which avoids anydamage being caused to wool or silk.

A still further object of this invention is to propose a novel processfor the dyeing of wool, silk or blends thereof with indigo, whichprovides a dark and level indigo shade.

Yet further object of this invention is to propose a novel process forthe dyeing of wool, silk or blends thereof with indigo, which providesadequate fastness.

Still a further object of this invention is to propose a novel processfor the dyeing of wool or silk or blends thereof with indigo, which isefficient.

DESCRIPTION OF THE INVENTION

It is known that alkali disrupts the disulphide bonds in wool andresults in the formation of thiol groups. Such a reaction is utilized inthe present invention in that before the labile thiol groups recombineor form fresh cross links, the thiol groups are, in situ, brought incontact with (leuco) indigo, some hydrogen bonds/salt linkages betweenthe dye and the thiol groups in the substrate are effected. Thus, thetotal dye-combining capacity of the substrate is increased by generatingadditional dye-combining groups or sites by the action of alkali onwool. The rate and extent of dye absorption is enhanced by virtue ofincreased accessibility of the fibre to the dye.

In accordance with this invention, a mordanting of wool and silk in theform of cationization on line enhances dye-fibre affinity by virtue ofimparted cationic dye-combining sites for indigo which is anionic inalkaline media. Such mordanting also reduces the heterogeneity in thesubstrate and offer thereby level dyeing.

Wool and silk are sensitive to alkali and might degrade significantlywhen in contact with alkali particularly caustic soda for anyappreciable length of time. In case of wool, such a normally negativepoint is instead taken advantage of by using generated thiol groups asadditional dye combining sites.

Thus, to overcome the disadvantages of the prior art, cationization ofwool and silk or deposition of insitu generated cationicpolymer/oligomer within fibre provides the required result in that thecationic charge in the substrate could attract the anionic leuco indigodye

According to this invention there is provided a process for thepretreatment of wool, silk or blends thereof comprising the step ofonline cationization of silk, wool or blends thereof to render itreceptive to indigo dyeing by imparting a cationic charge and generatingdye-combining sites.

The treatment comprises in the step of cationization by passing wool,silk or blends thereof through pretreatment baths and then subjecting tothe step of a passage in air, which is preferably online. The onlinepretreatment could be effected either at ambient temperature or atsub-ambient temperature, for example, at 12-15 Decree C. Thepretreatment bath comprises 3- to 120 gpl, and preferably 3 to 80 gpl ofcationizing compound and 3 to 25 gpl of caustic soda either separatelyor in admixture. Wool, silk or their blends is subjected to a treatmentin the pretreatment baths for 5 to 30 seconds each and to the step ofaeration for 30 to 180 seconds.

Reference is now made to the step of dyeing of silk or wool with indigosubsequent to the pretreatment step in a manner known in the art, butthe step of dyeing comprises in passing in tandem the pretreated silk,wool or blend through a plurality of indigo dye baths following thedip-squeeze-air (oxidation) steps.

In accordance with this invention, the dye bath contains (leuco) indigo,soda ash sodium hydroxide, sodium hydrosulphite and known chemicalauxiliaries. As described hereinabove, wool and silk have a tendency tolose its cationic charge in the presence of alkali. Simultaneously, theaddition of alkali to the dye bath is necessary in order to render thedye soluble. However, the pretreatment step renders silk or woolreceptive to anionic leuco dye by virtue of imparted cationization andgenerated dye combining sites in the substrate. Thus, each dye bathcontains 1.0 to 3.5 gpl indigo and upto 8 dye baths may be provided. Thepassage of wool or silk through each dye bath is for a period of 5 to 30seconds each followed by the step of squeez and oxidation for a periodof 30 to 180 seconds for each step of oxidation. The dye-bath recipecontains soda ash mostly and between 15-20 gpl, in place of causticsoda. The latter is used for fine tuning of dye bath pH. The causticsoda and the cationizing chemical in the pre-treatment are kept atsubambient treatment in accordance with on embodiment of this invention.This is to reduce the severity of the process and help substrate retainits original strength.

Reference is made to the online pretreatment step in distinction to abatch process. One of the aspects of the present invention resides inthe online pretreatment step which provides a short period of contact ofwool, silk of its blends with caustic soda and cationizing chemicals. Itis known that wool, silk or its blend is sensitive to caustic soda.Thus, one of the aspects of the present invention is to ensure that thewool, silk or its blends is not deteriorated or damaged by thepretreatment step, which is ensured by a reduced contact period with thecationizing chemicals by such an online step.

EXAMPLE

The wool yarn was subjected to online cationization. The original sampleas well as the cationized sample were then analyzed for their nitrogencontents by Kjeldahl method. The sample showed increase in nitrogencontent over the control by 0.35%.

The nitrogen content of the cationizing chemical viz.,3-chloro-2-hydroxy propyl trimethyl ammonium chloride is 9.18%.

The 0.35% increase in nitrogen content in cationized sample wouldtherefore mean 5.8 gram of cationizing chemical was added up to 100grams of wool.

The spectral study indicated no major change in the structural patternof wool.

The nitrogen increase in case of silk was 0.2% which means 3.3 gms ofcationizing chemical was deposited onto 100 gms of silk.

DESCRIPTION WITH REFERENCE TO ACCOMPANYING DRAWING

Reference is made to FIG. 1 of the accompanying drawing whichillustrates a schematic line diagram of a known rope dyeing apparatusbut modified to contain the pretreatment steps.

As shown in FIG. 1, the pretreatment apparatus contains Box 1 and Box 2with squeeze rollers SR1 therebetween.

First box Box 1 may contain cationizing chemical solution followed byaeration thereafter which allows time for cationizing chemical topenetrate well into wool/silk substrate.

The second box Box 2 may contain caustic soda solution also followed byaeration.

Aeration allows time for reaction between wool/silk and cationizingchemical in the presence of caustic soda. Alternatively, Box 1 maycontain both cationizing chemicals and caustic soda.

After impregnation of wool/silk with the said cationizing chemical, anaeration is effected which allows for the time for the reaction betweenwool/silk and cationizing chemical in the presence of caustic soda.

The second box Box 2 in such an instance could be empty-or could be usedfor a water-rinse

Indigo dyeing then follow in tandem, with conventional and known stepsof indigo dyeing, viz., dip-squeeze-air.

As shown in FIG. 1, the pretreated substrate then passes through aplurality of dye boxes, for example 1 to 8 dye boxes, followed each timeby aeration. The substrate is then subjected to post dyeing wash off inwash boxes and then passed through drying cans.

It is to be noted that the present invention is susceptible tomodifications, adaptations and changes by those skilled in the art. Suchvariant embodiments employing the concepts and features of thisinvention are intended to be within the scope the present invention,which is further set forth under the following claims:

1-8. (canceled)
 10. A process for on line pretreatment of a substratecomprising: providing a substrate, wherein the substrate is selectedfrom the group consisting of wool, silk, and blends thereof; andpretreating the substrate, wherein pretreatment of the substratecomprises generating dye combining sites by treatment with at leastcaustic soda and for a period sufficient to generate said sites andwithout damaging said substrate.
 11. The process of claim 10, whereinthe pretreatment step further includes the step of cationizing bypassing the substrate through a pretreatment bath followed by aerationto provide a dwell period for the pretreated substrate.
 12. The processof claim 11, wherein the pretreatment step comprises treatment withcaustic soda and a cationizing agent.
 13. The process of claim 10,wherein the step of pretreatment in the bath is carried out for 5 to 30seconds and aeration for a period of 30 to 180 seconds.
 14. The processof claim 11, wherein the pretreatment bath comprises 3 to 80 gallons perliter of a cationizing compound and 3 to 25 gallons per liter of causticsoda.
 15. The process of claim 11, wherein the pretreatment bathcomprises 3 to 25 gallons per liter of caustic soda.
 16. The process ofclaim 10, wherein pretreatment of the substrate is carried out atambient temperature.
 17. The process of claim 10, wherein pretreatmentof the substrate is carried out at 12-15° C.